The aircraft landing gear did not extend when the flight crew selected it down for landing at Ottawa because the circuit breaker for the electric motor that operated the hydraulic pump had opened. The landing gear will not extend by the normal method without the operation of the electric motor to operate the hydraulic pump. The depleted nitrogen charge in the landing gear accumulator would result in more frequent cycling of the hydraulic pump motor to maintain system hydraulic pressure. The combination of repeated landing gear cycling at North Bay and more frequent cycling of the electric hydraulic pump motor because of the depleted nitrogen charge in the hydraulic system accumulator likely caused the 60-amp circuit breaker to overheat and open, shutting off electric power to the hydraulic pump motor. Operation of the emergency landing gear extension system resulted in all three landing gear extending, but the right main landing gear did not lock in the down position and collapsed as the aircraft weight settled on it during the landing roll. It could not be determined why the right main landing gear did not lock in the down position during the emergency extension.Analysis The aircraft landing gear did not extend when the flight crew selected it down for landing at Ottawa because the circuit breaker for the electric motor that operated the hydraulic pump had opened. The landing gear will not extend by the normal method without the operation of the electric motor to operate the hydraulic pump. The depleted nitrogen charge in the landing gear accumulator would result in more frequent cycling of the hydraulic pump motor to maintain system hydraulic pressure. The combination of repeated landing gear cycling at North Bay and more frequent cycling of the electric hydraulic pump motor because of the depleted nitrogen charge in the hydraulic system accumulator likely caused the 60-amp circuit breaker to overheat and open, shutting off electric power to the hydraulic pump motor. Operation of the emergency landing gear extension system resulted in all three landing gear extending, but the right main landing gear did not lock in the down position and collapsed as the aircraft weight settled on it during the landing roll. It could not be determined why the right main landing gear did not lock in the down position during the emergency extension. The crew was certified and qualified for the flight. Aircraft records indicated the aircraft was certified and equipped in accordance with existing regulations and approved procedures. The 60-amp circuit breaker for the electric hydraulic pump motor opened, rendering the aircraft normal hydraulic landing gear system inoperable. The aircraft right main landing gear indicator did not indicate the down and locked position following extension of the aircraft landing gear by the emergency method. The aircraft right main landing gear collapsed during the landing roll. The aircraft landing gear hydraulic system accumulator was found defective and would not maintain the required nitrogen charge. The depleted landing gear hydraulic system accumulator should not affect the ability of the landing gear system to actuate in normal or emergency operation.Findings The crew was certified and qualified for the flight. Aircraft records indicated the aircraft was certified and equipped in accordance with existing regulations and approved procedures. The 60-amp circuit breaker for the electric hydraulic pump motor opened, rendering the aircraft normal hydraulic landing gear system inoperable. The aircraft right main landing gear indicator did not indicate the down and locked position following extension of the aircraft landing gear by the emergency method. The aircraft right main landing gear collapsed during the landing roll. The aircraft landing gear hydraulic system accumulator was found defective and would not maintain the required nitrogen charge. The depleted landing gear hydraulic system accumulator should not affect the ability of the landing gear system to actuate in normal or emergency operation. For undetermined reasons, the right main landing gear did not lock down following extension of the landing gear by the emergency method, causing it to collapse on the landing roll. The normal aircraft hydraulic landing gear extension and retraction system was rendered inoperable when the circuit breaker for the electric hydraulic pump motor opened, likely due to overheating from repeated cycling of the electric motor. A defective landing gear hydraulic system accumulator contributed to the repeated cycling of the electric hydraulic pump motor.Causes and Contributing Factors For undetermined reasons, the right main landing gear did not lock down following extension of the landing gear by the emergency method, causing it to collapse on the landing roll. The normal aircraft hydraulic landing gear extension and retraction system was rendered inoperable when the circuit breaker for the electric hydraulic pump motor opened, likely due to overheating from repeated cycling of the electric motor. A defective landing gear hydraulic system accumulator contributed to the repeated cycling of the electric hydraulic pump motor. Safety Action Actions Taken Since the occurrence Transport Canada, ASD, has taken the following actions: The Beech A100 King Air SOPs have been amended to allow a single, in-flight reset of the electric hydraulic pump motor 60-amp circuit breaker. The 60-amp circuit breaker in the accident aircraft has been relocated to the aircraft cockpit. This location is now the same as on the other Beech King Air aircraft in the ASD fleet that have the Aviadesign STC hydraulic landing gear. A mirror has been installed on the inboard side of the Beech A100 King Air engine cowlings to provide a means for pilots to observe the nose landing gear position from the aircraft cockpit. The ASD Flight Operations Manual was amended to read, where practicable, it is recommended that the pilot contact the applicable ASD operations centre, and state the nature of the problem, the assistance required, and the time remaining before a landing is necessary. Actions Required The safety actions taken by Transport Canada, ASD, have possible continuing airworthiness operational implications for the fleet of similar aircraft operating elsewhere in Canada and in other countries. These actions include permitting a single in-flight reset of the electric hydraulic pump motor 60-amp circuit breaker, relocating the 60-amp circuit breaker to the cockpit, and installing a mirror to provide a means for the pilot to observe the nose landing gear position from the cockpit. Dissemination of this information to other King Air operators in Canada and around the world for the purpose of possible similar safety actions by other operators would reduce the risk of similar accidents. Annex 8, Airworthiness of Aircraft, to the Convention on International Civil Aviation contains provisions urging the State of Registry of an aircraft to notify the State of Design about information that might cause adverse effects on the continuing airworthiness of an aircraft. It seems appropriate that the changes to the configuration and operating procedures for the Transport Canada King Air fleet should be brought to the attention of the Federal Aviation Administration in the United States, the State of Design, for possible fleet-wide safety action. Therefore, the Board recommends that: